Bloom syndrome (BS) is an autosomal recessive disorder characterized by proportional dwarfism, dysmorphic facial features, photosensitive skin rash, immunodeficiency and the development of a wide variety of malignancies. Cytogenetic studies have shown BS patients to have an increased rate of chromosome breakage and a characteristic elevation of sister chromatid exchange to levels approximately tenfold higher than that seen in normal individuals. Although a link has not been formally established, it is believed that chromosomal instability is responsible for the frequent occurrence of cancer in BS homozygotes and BS has been broadly classified as a DNA repair disorder. Several forms of genetic analysis have mapped the gene responsible for BS to the distal part of chromosome 15 in the vicinity of fes at 15q26.1. As a preliminary step to the cloning of the BS gene, a set of yeast artificial chromosomes (YACs) mapping to 15q26.1 were isolated. End fragments from some of these YACs were subcloned using a vectorette end-rescue technique. Sequence analysis of these end clones was used to develop oligonucleotide primer pairs that were then used to screen YAC libraries for overlapping YAC clones. The strategy being pursued was aimed at developing an extensive set of overlapping YACs that covered the BS region of 15q26.1. Analysis of one of the YAC end clones showed it to contain a region of strong homology to a family of genes related to the Saccharomyces cerevesiae gene snf2. Members of this gene family have been shown to have a variety of functions including transcriptional regulation, control of chromatin structure and DNA repair. The latter group of genes includes the yeast RAD16 and RAD54 proteins and the human gene ERCC6. Cockayne syndrome, complementation group B, has recently been found to be due to mutations in ERCC6. This suggested that the gene identified by the genomic fragment cloned in the YAC end should be regarded as a candidate for the BS gene. An overlapping set of cDNA clones was assembled by cDNA library screening, cloning of RT-PCR products and search of the dbEST database maintained by NCBI. Sequence analysis of these clones revealed the presence of portions of three different transcription units with highly conserved sequences in the domain showing strong similarity to the SNF2 family of proteins and less similar sequences elsewhere. Mapping of the three transcripts confirmed that the gene corresponding to the YAC end was located in 15q26.1 while the other genes were on chromosomes 5 and 17. Subchromosomal localization studies have not yet been performed for the latter two genes. Efforts are currently underway to obtain DNA sequence corresponding to the complete open reading frames of the three snf2-related genes that have been identified in this work. Experiments are also in progress to determine whether mRNA from BS fibroblast cell lines might have mutations in the chromosome 15 gene.